Fats and oils are universally used throughout the world as food substances, and in fact, they constitute approximately 25% to 50% of the caloric intake of mankind. Such materials comprise esters of glycerol with a variety of fatty acids, termed glycerides, and as such, the materials are characterized both by their ester linkages, and by their degree of unsaturation. Fats, which are differentiated from oils solely by the fact that fats are solid, or semisolid at room temperatures, whereas oils are liquids, occur in fruits and seeds of plants, as well as in animal tissue. The higher fatty acids with which the glycerol portion is esterified comprise both saturated and unsaturated acids. Saturated fatty acids tend to result in glycerides having higher melting points, as do fatty acids of higher molecular weight. Unsaturated fatty acids, usually vegetable-derived, and the lower molecular weight fatty acids normally have lower melting points.
Due in part to their relatively high amounts of unsaturates, however, liquid edible oils are particularly susceptible to rancidity caused by oxidation which produces a complex mixture of volatile aldehydes, ketones, and acids. While the oxidative process can be retarded to some extent, for example, by stabilization with antioxidants which inhibit oxidation, solid fats in many instances enjoy greater popularity, not only because they have a relatively long shelf life, but also because they are capable of repeated heating without undergoing oxidative degradation. Solid fats can be shipped more readily, and they are also adapted to uses where only solid fats can be used, for instance, as spreads for breadstuffs and the like.
In the past, solid triglycerides have been produced by the hydrogenation of oils carried out commercially, for example, by bubbling hydrogen gas through oils containing a suspension of finely divided nickel metal. In the process, the double bonds of oleic, linoleic, linolenic, and eleostearic glycerides in the oil are converted into the hard, waxy tristearin. By controlling the amount of hydrogen added, solids of different consistencies may thus be obtained.
In recent year, however, it has been discovered that in humans, saturated triglycerides are converted into cholesterol that is subsequently deposited as plaque on the walls of blood vessels. The deposition of plaque results in the constriction of the blood vessels, making them more susceptible to blockage caused by the clotting which occurs from time-to-time in the bloodstream. Sudden blockage of these constrictions by clots is a major cause of frequently fatal heart attacks and strokes throughout the world. For this reason, as well as its cost, hydrogenation processing of the type described is now frequently recognized to be undesirable.
In an effort to avoid such disadvantages, transesterification, sometimes termed interesterification, has been suggested as a method for converting liquid oils into solids. In the past, transesterifications have been carried out under anhydrous conditions using sodium methoxide as a catalyst. In the presence of methoxide ions, the various acids attached by ester linkages to the glycerol are randomly exchanged between the triglyceride molecules very rapidly. Since the acids are evenly distributed in nature, treatment with methoxides provides a random distribution of the acids among the triglycerides, resulting in the formation of some that are solid.
Since removal of one of the products of a reversible reaction results in an equilibrium shift, removal of the solid triglycerides by means of selective crystallization causes the reaction to generate still more of the removed component. The process is sometimes referred to as a "directed" interesterification. Directed interesterification processes can be used to produce triglyceride mixtures having an extensive solid content, from liquid oils, without destroying the double bond initially present in the starting material.
Unfortunately however, methoxide catalysts commonly show a tendency to interact with the triglycerides with which they are employed, producing methyl esters which can hydrolyze to form the poisonous methyl alcohol.
To avoid this problem, a different process has been proposed in Indian patent 160,248. There, the process of converting edible oils into solid fats without hydrogenation is accomplished by treating the oils with sodium propylene glycoxide. This catalyst, which produces the desired transesterification, eliminates the possible formation of deleterious substances, and is readily removed following the transesterification by a simple water treatment.
Although the transesterification thus achieved produces the desired solid triglyceride product, the process would be even more desirable if the reaction rate could be accelerated.